Fixed #29538 -- Added support for ordering related objects by query expressions.

[alexandrelaplante]

Added support for ordering related objects by query expressions

https://code.djangoproject.com/ticket/29538

[alexandrelaplante]

Not sure what the expected behavior here is.

class Album:
    class Meta:
        ordering = [models.F('artist').asc(nulls_last=True)]

class Album:
    class Meta:
        ordering = ['artist']

Should both of these work the same way? Or does
the first mean "order by artist_id"
and the second mean "order by the default order of Musician"?

[carltongibson]

Pretty sureordering = ['artist'] will mean order by artist (rather than the default order of the related model). (In the compiler, field names get converted to column references, rather than resolved via related models' Meta...)

Possible to check that though...

[charettes]

@carltongibson no, you should useartist_id if you want to achieve that. There's a documented difference between order_by('artist') and order_by('artist_id') as it was not even possible to disable related ordering before 24ec953.

Given Artist.Meta.ordering = ['foo', F('bar').asc(nulls_last=False)]

then Album.Meta.ordering = [models.F('artist').asc(nulls_last=True)]should be the equivalent of

[F('artist__foo').asc(nulls_last=True), F('artist__bar').asc(nulls_last=True)]

[carltongibson]

Good. Thanks Simon.

[orf]

What is this for? Is the filter() necessary? Couldn't this just be inlined below?

[alexandrelaplante]

I think this is the alternative

if prefix is None:
    new_prefix = name
else:
    new_prefix = prefix + LOOKUP_SEP + name

prefix is None at the top of the recursive call.

Which is better?

[carltongibson]

Hi @alexandrelaplante.

Thanks for the input here. Sorry for the slowish pick-up.

One initial point on the placement of the tests:

Can we include these in the existing ordering app?

• We try not to add new models if we can avoid it. (They slow everything down.) Can we re-use or adjust the ordering.models models?
• Can we move order_with_expression.tests into ordering too? (Either in a test case in tests.py or a separate test_....py file if your prefer.)

[alexandrelaplante]

@carltongibson I tried reusing the models at first but they weren't quite structured in the right way for my tests. I'll give it another try, possibly restructuring the existing tests to accommodate.

[carltongibson]

@alexandrelaplante Thanks! (Perhaps if you move the tests all into ordering first, we can see if we can remove the new models second.)

[auvipy]

please rebase and address carls comments

[felixxm]

Closing due to inactivity.